Boiling Phenomenon and Bubble Formation CFD Tutorial

Boiling Phenomenon and Bubble Formation, Ansys Fluent CFD Simulation Training

Boiling is the fast evaporation of a liquid when heated to its boiling point. The temperature at which the liquid’s vapor pressure equals the pressure exerted on the liquid by the surrounding environment. Boiling may be classified into two types: nucleate boiling, in which tiny bubbles of vapor develop at distinct places, and critical heat flux boiling, in which the boiling surface is heated above a specific critical temperature and a film of vapor forms on the surface. Transition boiling is an unstable intermediate form of boiling that has components of both forms. The boiling point of water is 100 °C or 212 °F; however, it is lower at higher altitudes due to reduced air pressure. Boiling Phenomenon and Bubble Formation CFD simulation by ANSYS Fluent is carried out in this product.

Project Description

In this simulation, the difference between HRIC-compressive-Geo-reconstruct models and their effect on the output results were discussed by changing the volume fraction discretization. The computational domain is a 15 x 15 cm cube with a 2 x 2 cm heated surface. The change in volume fraction discretization showed no significant change in surface temperature and heat transfer coefficient. Only the Geo-Reconstruct model visually models a more realistic simulation with a higher computational cost.

Geometry & Mesh

The computational domain was designed using Design Modeler software. The computational domain includes a 15 x 15 cube with a 2 x 2 cm heating surface.

Ansys Meshing software was used for grid generation, and the type of problem elements was structured. Also, the total number of elements was about 570000.

CFD Simulation Solver Setting

In this simulation, the following hypotheses are established:

• A pressure solver was used.
• The problem was solved transient
• An explicit VOF multiphase model was activated.
• Gravitational and capillary effects were considered.

Also, the table below shows the characteristics and values of boundary conditions, along with the models and hypotheses.

Material Properties
Water
Amount		Fluid properties
998.2		Density (kg/m3)
Water vapor
Amount		Fluid properties
0.5542		Density (kg/m3)
Multiphase
Homogeneous model		VOF (volume of fluid)
Number of Eulerian phases		2 (water-water vapor)
Interface modeling		sharp
Formulation		Explicit
Phase interaction
Evaporation & Condensation		Lee model
From phase Frequency (1/s)		10
Fluid type		Surface tension coefficient (N/m)
Water-vapor		0.06
Boundary Condition
Type		Heat flux (W/m^2)
Hot wall		10000
Cell zone condition
Fluid		mixture
Turbulence models
K-	viscous model
Realizable	K- model
Standard wall function	Wall function
Solution methods
Coupled			pressure velocity coupling
Modified body force weighted	pressure		spatial discretization
First-order upwind	momentum
First-order upwind	turbulent kinetic energy
First-order upwind	turbulent dissipation rate
Volume fraction	Geo-reconstruct Compressive Modified HRIC
Initialization (Standard)
0 (Pa)		gauge pressure
373.15 K		Temperature
1		Water volume fraction

Boiling Phenomenon and Bubble Formation Results

The table below clarifies that the average surface temperature at the input of the problem and the heat transfer coefficient in these three discretization models do not change much. In the simulation with high computational cost, it is suggested that the discretization methods of compressive and modified HRIC Be have a lower computational cost. But for near-reality simulation, it is better to use the Geo-reconstruct model, which has a higher computational cost.

Discretization method	Surface temperature (K)	Heat transfer coefficient (W/m^2.K)
compressive	385.835	102.398
Modified HRIC	385.688	102.547
Geo-reconstruct	387.96463	101.76327

The grid elements need to be finer near the hot surface to increase the simulation accuracy. The distance of the first cell is essential in boiling studies, but this is very effective in increasing the computational costs in 3D simulations.